Pseudorandom noise (PRN) codes are an important element of code division multiple access (CDMA) based satellite navigation systems. Each satellite within a GNSS constellation has a unique PRN code that it transmits as part of the C/A navigation message. This code allows any receiver to identify exactly which satellite(s) it is receiving.
The PRN codes act as spreading codes in the spread-spectrum communications system, and must be carefully chosen to minimise interference between each satellite signal. Failure to do so would leave the system open to so-called CDMA noise, potentially degrading performance to unworkable levels.
It is not only satellites that are allocated PRN codes, they are also necessary for augmentation systems and pseudolites. Therefore, the PRN codes for each GNSS have to be carefully managed.
In the GPS system, this management is performed by the GPS Directorate, which has already defined a large set of GPS PRN sequences that provide good auto- and cross-correlation properties. Operators of augmentation systems and other pseudolites must then apply to the GPS Directorate to be allocated one of the codes from this sequence.
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